My stepmother is a podiatrist and when DH had a crack in his heel she told him to use DS' Bepanthen on it to help heal the crack! It worked too.

In terms of maintaining your feet once you've got the crack to heal, I would file off the rough patches, slather your feet in Vaseline and put on cotton socks and sleep with the socks on overnight. Very old-fashioned but works really well. When you've got them under control you should be able to maintain them with a daily application of Eulactol or some other heel balm and just do a Vaseline and cotton socks booster once a week, say.